Suggestion：support PLC Type Select

Siemens have some different Device Model ，such s7200 ，s7100，s71200,s71500 ，etc. But their S7 protocol is not common。Can support enum to select？ 

- Emum:
```go
{
    S7200 = 0,
    S7300 = 1,
    S7400 = 2,
    S71200 = 3,
    S71500 = 4,
}
```
- Example
```go
func NewTCPClientHandler(model int, address string, rack int, slot int) *TCPClientHandler
```
